function vs:chain/pop/title_modify with storage vs:system chain.subtitle[-1]
$title @a title {"nbt": "chain.subtitle[$(index)]","storage": "vs:system","interpret": true}

data remove storage vs:system chain.subtitle[-1]

title @a subtitle {"nbt": "chain.subtitle","storage": "vs:system","interpret": true}
execute as @a at @s run playsound block.chain.place master